<html maaa=a >
        <head>
          <style>
            #container{
              width: 500px;
              height: 300px;
              display: flex;
              background-color: rgb(255,255,255);
            }
            #container #myid{
              width: 200px;
              height: 100px;
              background-color: rgb(255,0,0);
            }
            #container .a1{
              flex: 1;
              background-color: rgb(0,255,0);
            }
          </style>
        </head>
        <body>
          <div id="container">
            <div id="myid"></div>
            <div class="a1"></div>
          </div>
        </body>
        </html>